High efficiency magnetron



Feb. 6, 1951 P. L. SPENCER HIGH EFFICIENCY MAGNETRON 2 Sheets-Sheet 1 Original Filed Jan. 25, 1945 IH/ -l lllllli INVEN7OR PERCY L.SPs-cs/2 BY QQM Ar Nev Feb. 6, 1951 SPENCER Re. 23,337

HIGH EFFICIENCY MAGNETRON Original Filed Jan. 25, 1943 2 Sheets-Sheet 2 vawrorz PERCY L. SPENCER BY 52% TQ'ORNE Y Reissued Feb. 6, 1 951 .HIGH EFFICIENCY MAGNETRON Percy L. Spencer, West Newton, Mass., assignor to Raytheon Manufacturing Company, West N ewton, Mass, a corporation of Delaware Original No. 2,410,396, dated October 29, 1946, Se-

rial No.473,557, January 35, 1943. Application for reissue June 15, 1950, Serial No. 168,321

Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue 2 Claims.

This invention relates to a magnetron, and more particularly to a magnetron which generates oscillations within one or more cavities formed within its anode structure, and in which the frequency of said oscillations is determined primarily by the geometry of said cavities.

With prior devices of the above type, where a plurality of oscillating cavities were used and the oscillations were led off through coupling means associated with one of said cavities, energy was fed from one cavity to the next with subsequent diificulty, resulting in a decrease in efficiency. Also said prior devices were very critical as to the spacing between the side walls at the entrance to the cavity, thus introducing the problem of extensive and time-consuming machine work and constructing such cavities within the necessary critical tolerances.

An object of the present invention is to increase the ease of transfer of energy from one oscillating anode cavity to another in a plural cavity magnetron.

A further object is to decrease the effect-of the spacing between the anode arms at the entrance to the anode cavity on thegenerated frequency.

A still further object is to contribute to each of the foregoing objects by increasing the ratio between the inductance and capacity of each oscillating cavity within certain limits.

The foregoing and other objects of this invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying'drawings, where- Fig. 1 is a transverse section of a magnetron embodying my invention, taken along line ll of Fig. 2;

Fig. 2 is a longitudinal cross-section taken along line 22 of Fig. 1;

Fig. 3 is a cross-section similar to that of Fig. 1 of another embodiment of my invention taken along line 3-3 of Fig. 4; and

Fig. 4 is a longitudinal cross-section taken along line 44 of Fig. 3.

The magnetron illustrated in Figs. 1 and 2 comprises an envelope I which is preferably made of a block of conducting material, such as copper. This block forms the anode structure of the ma netron. The block has hollow end sections which are covered by caps 2 and 3, likewise of conductive material, such as copper. Between the hollow end sections of the block I is located a centrallybridging portion 4. The portion 4 is provided with a central bore 5 within which is supported,

"substantially at the center thereof, a cathode 6 which may be of the indirectly-heated oxidecoated thermionic type. Light conducting shields T and 8 may be supported adjacent the upper and lower ends of the cathode 6 so as to prevent electron beams from being projected out toward the end caps 2 and 3. Thecathode is supported .by a pair of cathode lead-in conductors 9 and i0 sealed through theglass seals H and i2 mounted at the outer ends of pipes l3 and I4 hermetically fastened within-the walls of the block I adjacent the upper and lower hollow-end sections. A plurality of cavities I5 is formed-in the bridging portion i, and extend radially from the central bore 5. Eachcavity [5 possesses a pair of side arms 58, the arms l6 betweenadjacent cavities serving jointly as one of the side arms of said adjacent cavities. The forward faces of the arms l6 form electron-receiving anode segments with relatively small spacing between each arm, thus presenting a plurality of slots to the electron stream emanating from the cathode 6.

Heretofore the side arms of each anode cavity of a plural cavity magnetron have extended back from the entrance slots substantially parallel with each other. However, the operation of such a device is improved when the side arms of each anode cavity 15 diverge from each other, preferably to as great an extent as possible. As shown in Fig. 1, the side arms l6 diverge in this way substantially to the rear of their associated cavities, where the back wall of each cavity extends around to join the two side walls to each other. This divergence is attained by providing each side arm IE with straight side walls which converge from the inner end of said arm toward the rearof the associated cavity 15 for an appreciable distance.

The frequency of the oscillations generated by each anode cavity [5 is determined primarily by the space between the side walls of the cavity and the size of the cavity loop. Diverging the side arms of the cavity, as described above, does two things. First, by introducing a greater separation between the side walls of the cavity, the capacity is decreased;-and second, by increasing the transverse area of the cavity, the i1 stance thereof is increased. For a predetermi ed reso nant frequency, theproduct of capacityand irrductanceis a constant. In accordance with my invention, the indu ctance may be increased rela tiv e'to the decrease incapacity so that the resonant frequency of mydevice remains substantially the same as that of prior devices of similar size. According to well-known circuit theory at lower wave lengths,

where f=frequency of the oscillations produced L=inductance of the oscillating circuit, and

R=eifective resistance of the oscillating circuit to the oscillatory currents.

By analogy with the above theory, we may consider that the inductance of each oscillatingloop in my ultra-high frequency tube is increased enough to raise the Q of my system to a considerable degree. The higher'the Q of such a device as I have disclosed becomes, the less will be the influence exerted by changes in voltage and electronic conditions, such as cathode emission, upon the frequency at which the device operates. Since under normal practical operating conditions, changes of the above nature inevitably occur, the increased Q of my device results in operation with a much more constant frequency.

By constructing each anode oscillating loop with a relatively small gap between the ends of the side arms and having those arms diverge sharply from the gap as above described, I have found that the improvements due to decreased capacity, increased inductance and correspondingly increased values of Q readily can be obtained. If an attempt were made to secure these improvements by increasing the size of the gap at the entrance to the oscillating loop, several adverse effects would be produced. With such an enlarged .gap, in order to produce the desired frequency, the physical size of the loop would have to be increased to such an extent as to unduly enlarge the overall tube dimensions. Also the-propagation of energy from one oscillating loop to the next would be slowed down, introducing relatively large phase differences between various parts of the tube together with the creation of the probability of the tube oscillating in undesired modes at undesired spurious frequencies. In magnetrons such as I have described, the electrons describe. orbits about the cathode. For maximum effectiveness, the circular component of electronic speed around the cathode adjacent the anode surfaces should be substantially equal to the rate at which the generated wave is propagated from loop to loop around the tube. Thus the slowing down of the propagated Wave which would result from an increase in the size of the entrance gap to each oscillating loop would tend to upset this desired condition 'of equality, and thus decrease the effectiveness of operation of the tube. The most satisfactory spacing between the side arms of each oscillating loop at the entrance gap thereto is of the order of that at which the transit time of electrons passing across the gap is less than about a quarter of a cycle of the oscillations which the tube .is adapted to generate. Due to diverging arrangements of the anode arms in my construction, slight variations in the spacing between the ends of the arms produce smaller changes in capacity than in the case of previous constructions. Therefore, the over-all effect of my novel construction is to produce a device in which the spacing between the anode arms is not critical, and in which the tolerances of any machine work which might be necessary are substantially increased.

As already pointed out, my structure produces an increase in the transverse cross-sectional opening of each loop [5. I have found that in addition to this factor, increasing the inductance of the cavity, the ease of transfer of energy from one oscillating loop to the next is substantially increased, thus producing an increase in efiiciency of the device.

When a magnetron such as I have described above is placed between suitable magnetic poles I1 and I8 to create a longitudinal magnetic field and the device is energized, oscillations are set up in each of the oscillating loops IS. The oscillatory energy thus generated in each of the loops I5 is readily propagated through the tube by transfer from one loop to the next, so that this energy may be led out readily from the tube by means of a coupling loop l9 extending into one of the oscillating loops [5 and having one end thereof fastened to the inner wall of said oscillating loop. The other end of the coupling loop [9 is connected to a lead wire 20 which passes through a glass seal 2| mounted at the outer end of a pipe 22 likewise hermetically fastened through the wall of the envelope I. An

additional conducting pipe, not shown, may be electrically connected to the pipe 22, and forms with the wire 20 a concentric line through which the high frequency oscillations generated by the magnetron may be conducted to a suitable utilization circuit. 1

Since the spacing between the ends of the anode arms which forms the gap at the entrance to each oscillating cavity is not critical, my invention lends itself to constructions in which fine machine work is completely eliminated and which may be assembled from approximately machined parts and stamped members. Such an arrangement is shown in Figs. 3 and 4. The device shown in these figures comprises an envelope 3| which is likewise made of a block of conducting material, such as copper. The ends of they block are covered by caps 32 and 33 likewise formed of conductive material, such as copper. The envelope 3| is formed with a central annular projection 34 in which are cut a number of longitudinal slots 35. The machine work necessary for the formation of the envelope 3| and the slots 35 need not be of a very fine nature, since slight variations in the dimensions of the resulting structure do not have very marked effects on the operation of the device. A plurality of radial plates 36 is fastened into the slots 35, preferably by being soldered therein with a silver solder. The plates 36 may be stamped out of a sheet of highly conductive copper. The plates 33 thus form side arms of a plurality of oscillating anode cavities similar to those described in connection with Fig. 1. The inner ends of the plates 36 form anode faces which cooperate with a'cathode 31 supported substantially centrally of said anode faces. The cathode 37 may be of a type similar to the catrode 6 of Fig. 1. Here likewise light conducting shields 38 and 39 may be supported adjacent the upper and lower ends of the cathode 31. The cathode itself is supported by a pair of cathode lead-in conductors 4G and 4| which pass out from the envelope 3% through pipes 42 and 43 in a manner similar to that described in connection with the lead-in conductors 9 and I0 of the arrangement in Figs. 1 and 2. The oscillations which are up within the device shown in Figs. 3 and 4 may be led out from the tube by means of a coupling loop 44 connected to a lead wire 45 which passes from the tube through a pipe 46 in a manner similar to that described in connection with the loop I! and conductor 2B of the arrangement shown in Figs. 1 and 2.

It will be seen that the point at which the plates 36 approach each other most closely forms the entrance gap to each of the oscillating anode loops, which gap is presented to the electron stream emanating from the cathode 31. Furthermore the side walls of each of the oscillating gaps diverge from each other from the entrance gap as described in connection with the arrangement shown in Figs. 1 and 2, and thus the arrangement of Figs. 3 and 4 possesses the advantages of my invention as already described.

Of course it is to be understood that this invention is not limited to the particular details as described above as many equivalents will suggest themselves to those skilled in the art.

What is claimed is:

1. An anode structure for an electron-discharge device of the magnetron type comprising: a cylindrical body provided with a plurality of inwardly-directed, radially-disposed vanes; each aid vanes having straight side walls which erge from the inner ends thereof toward said ewndrical body for an appreciable distance from said inner ends; adjacent vanes being connected to; each other through said cylindrical body, and forming with the connecting portion of said body, ajfcavity resonator.

1 2. An electron-discharge device comprising. a cathode; and an anode structure, spaced from said cathode, and including a plurality of anode hers; each pair of adjacent anode members, er with that portion of said anode strucying therebetween, defining a cavity reson- PERCY L. SPENCER.

No references cited. 

